1. Field of the Invention
The present invention relates to a ring-type local area network (LAN) which conducts data transmission among a plurality of communication stations which are connected each other in a ring form by means of a transmission path.
2. Prior Art
Conventionally, in cases in which music was performed in a music production studio and a music source thus created, audio data, resulting from the digital conversion of the sound produced by human voices or acoustical musical instruments, and performance control data, such as key-on signals or key-off signals generated by an electronic musical instrument such as a synthesizer or the like, were handled separately.
This was based on the following reasons.
First, performance control data, for example, key-on signals or key-off signals, are preferably transmitted as fixed length signals at the signal generating timing, so that it was possible to use packet switching type LANs, which have been conventionally used in the transmission of computer data. Accordingly, data transmission is sufficiently performed with only one transmission cable.
In contrast, stream data, such as audio data, which are in the form of a continuous time series, must be continually sent as data having a previously set length, for example, 24-bit data, and at a previously set sampling timing. This presents no problem in the case in which the transmission speed is sufficiently high and there are few data. However, there is a problem in the case in which a plurality of circuits overlap. For example, audio data have a comparatively low transmission speed; however, the permissible amount of transmission delay is extremely small, so that it becomes impossible to transmit audio data in real time. Accordingly, it is necessary to provide separate circuits dedicated to each audio data, and conventionally, it was necessary that the number of transmission cables used for audio data be equal to or greater than the number of sources.
Furthermore, software which is used for music production has recently begun to appear which is able to handle not only performance control data, but is also able to directly handle audio data.
Furthermore, in recent years, multimedia has come into increasing prominence. Multimedia technology involves the synchronized handling of mutually associated commands, which permit the processing by present-day computers of numerical values, characters, and graphics, and continuous time series stream data, such as audio data or picture data or the like. In addition, by using this multimedia in a computer, images or sounds can be added in the computer and a "feeling of reality" or "ease of use" can be provided which cannot be obtained by books or television. In addition, it is necessary to be able to simultaneously transmit commands and stream data in the case of this multimedia as well.
As a result of the various conditions stated above, it is desirable to be able to transmit both various types of stream data, which are represented by audio data, and commands which are represented by performance control data, on one LAN in real time without delay.
The transmission speed of commands in a packet switching type LAN was conventionally roughly 4-10 Mbps, however, the transmission speed of stream data, and especially picture data, is necessarily roughly 100 Mbps. Furthermore, the transmission speed of audio data is comparatively low; however, the permissible amount of transmission delay is extremely small. Thus, stream data has various characteristics which differ from those of commands.
Accordingly, the following methods have been suggested for the transmission of commands and stream data having different characteristics on one LAN.
(1) There is a method in which a plurality of protocols are provided by means of multiple frequency division. In other words, one transmission path having a certain frequency band width is divided into a plurality of frequency bands, and channels are allocated thereto and commands or stream data are transmitted on each channel by different protocols. For example, commands are transmitted by means of a certain protocol on the 1-100 Hz frequency band width channel and stream data are transmitted by means of a different protocol on the 101-200 Hz frequency band width channel.
(2) There is a method in which stream data commands are transmitted by means of multiple time sharing. In other words, this is a method in which circuit switching and packet switching are simultaneously conducted on one transmission path according to the type of data.
(3) There is a method in which stream data and commands are handled on the same protocol using ATM (asynchronous transmission mode). In this method, audio data, image data, commands, and the like, are all divided into fixed length blocks, termed cells, and are sent on the transmission path.
However, in the conventional LAN described above, there are the following drawbacks.
In method (1), it is necessary to prepare modems for each frequency, and thus the scale of the circuitry is large.
Furthermore, in method (2), the scale of the circuitry is large and transmission efficiency is also poor.
In addition, in method (3), as the exchangers distribute the cells at high speed by means of hardware, the scale of the circuitry is large.
In the conventional LANs described above, in cases in which difficulties occur, such as the interruption of the transmission path or breakdowns in the communication stations which control transmission, communications are interrupted, so that it is necessary to detect faults more quickly and to conduct repairs directly.
Circuit switching type LANs and packet switching type LANs have fault repair functions, so that the transmission of stream data and commands can be conducted without breakdowns.
However, in the methods (1).about.(3) described above, for the transmission of stream data and commands by means of one LAN, it was impossible to transmit stream data and commands on one LAN by means of a simple structure and by means of protocols, without a reduction in transmission efficiency.
Furthermore in cases in which faults occurred, it was impossible to repair such faults directly.